The use of alkali metal halides in sigle crystal form for the manufacture of laser windows is known. In addition, it has been established that for maximum efficiency the alkali metal halide should be as pure as possible. Even materials having purities of 99.999% form windows which have an undesirable tendency to absorb energy from the laser beam. This causes the window to overheat which in turn will cause it to fracture and become comparatively opaque.
Alkali metal halides have been purified by numerous prior art methods. Most of these methods involve processes incapable of producing material of the purity required for use in the laser technology. For example, French patent 1,326,657, dated Apr. 1, 1963, was directed to a process for rectification of fused salt baths used in a steel treatment process. An inert carrier is used as a vehicle for the reactive agents, SiCl.sub.4 or TiCl.sub.4, which getters the oxide impurity, causing a decarbonization of the steel being processed. The resultant metal oxide, SiO.sub.2 or TiO.sub.2 precipitates out and settles as mud at the bottom. No extension of this process could lead to ultra pure metal halide materials. The melt is saturated with oxide with respect to the metal oxide ingredients of the mud and there is no significant liberation of free halogens which act as purifying agents.
U.S. Pat. No. 2,770,526, by Lander, teaches a Method for Purifying Metal Halides for use in the production of a ductile titanium metal. Lander causes molecular chlorine to react with metal ion impurities in CaCl.sub.2 via the dissociation reaction of CCl.sub.4 shown below. EQU CCl.sub.4 .revreaction. C + 2Cl.sub.2 ( 1)
This process is inapplicable in the removal of cation and anion impurities from alkali metal halides. Molecular chlorine is not very effective for removing anion impurities such as hydroxides and oxides and the process does not favor production of nascent chlorine. In addition, carbon, a product of the reaction taught by Lander, is an undesirable complication in crystal growth.
U.S. Pat. No. 3,565,700, by C. B. Root, dated Feb. 23, 1971, discloses a method for preparing and purifying pure dry fluoride materials. Fluorine gas is passed through the fluoride materials in a refractory oxide material rotating chamber. Root does not achieve the degree of dryness required to remove OH.sup.- from a metal halide. Column 1, lines 53-56, show that Root is not concerned with sources of H.sub.2 O in the system. further, Root replaces the flow of the reactive atmosphere in his chamber with an inert gas to allow the materials to cool. This process coupled with the apparatus seal off step (Column 4, lines 2-3), results in a rehydrolysis of the purified materials. Rehydrolysis during the purification process precludes one from obtaining materials free of OH.sup.- impurities as needed in laser applications.
British patent 1,123,991, dated Aug. 1968, discloses a method of preparation of pure alkali metal halides for the production of single crystals. The process is a conversion process which generally proceeds according to the following reaction. EQU Na.sub.2 SO.sub.4 + C + Cl.sub.2 .fwdarw. 2NaCl + CO.sub.2 + SO.sub.2 ( 2)
carbon is used solely as a reactant to oxyanions functioning as a reductant. In our application, where purification of, and not conversion to the halide is achieved, fine carbon is not desired. The preparation of an alkali metal-carbon powder mixture in order to purify the alkali metal halide is both cumbersome and counterproductive. Other disadvantages of the "991" teachings are the use of a static atmosphere of gaseous halogen and the potential for rehydrolysis during the sealing of the reaction ampule and removal of the gaseous halogen.
British Pat. No. 1,137,582, dated Dec. 1968, also teaches a method for producing and/or refining of Alkali Metal Halides. It, too, involves multiple steps which result in the rehydrolysis of the product. In this patent, a halogen source is entrained in a carrier gas. Entrainment leads to a deposit of carbon in the final product, thereby reducing the purity level to an undesirable point. Following the treatment of the metal halide at an elevated temperature, 582 teaches the removal of the entrained halogen source and subsequent sealing of the reaction ampule. Applicants disregard these teachings because they considered them to produce undesirable results. When glass containing absorbed moisture is reheated to its melting point, as needed to seal the glass ampule (Page 2, lines 93-98 of 582), the absorbed moisture will be released and cause rehydrolysis of the purified materials in the absence of a continuous stream of gas acting as a "moisture getter."
While each of the above cited prior art references deal in some manner with purification processes for metal halides, they do not yield materials as acceptable for use in laser technologies. Impurity levels below that detectable by conventional analysis techniques show up as major problems when the materials are subject to high power laser applications. As stated before, unmeasurable OH.sup.- present in alkali metal halides absorbs laser energies and renders these materials less efficient. Applicants have found a way to eliminate these impurities and overcome the disadvantages of the piror art processes.